Construction machines such as, for example, dozers, loaders, excavators, motor graders, and other types of heavy machinery use one or more hydraulic actuators to accomplish a variety of tasks. These actuators are fluidly connected to a pump on the construction machine that provides pressurized fluid to chambers within the actuators. As the pressurized fluid moves into or through the chambers, the pressure of the fluid acts on hydraulic surfaces of the chambers to effect movement of the actuator and a connected work tool. When the pressurized fluid is drained from the chambers it is returned to a low pressure sump on the construction machine.
One problem associated with this type of hydraulic arrangement involves efficiency. In particular, the fluid draining from the actuator chambers to the sump has a pressure greater than the pressure of the fluid already within the sump. As a result, the higher pressure fluid draining into the sump still contains some energy that is wasted upon entering the low pressure sump. This wasted energy reduces the efficiency of the hydraulic system. In addition, the fluid emptying to the low pressure reservoir is passed through a throttle valve to control a lowering or retracting speed of the actuator. Throttling the fluid also results in a loss or waste of energy and undesired heating of the hydraulic fluid.
Some attempts have been made to recover this otherwise wasted energy. For example, U.S. Pat. No. 6,584,769 (the '769 patent), issued to Bruun on Jul. 1, 2003, discloses a hydraulic circuit including an engine, three hydraulic pumps, an accumulator, a double-acting hydraulic cylinder, and several associated control valves. The first of the three pumps can be used to extend and retract the hydraulic cylinder in a normal manner, in which energy stored in the hydraulic fluid discharged from the cylinder is lost. A second of the three pumps is connected to the engine and, along with the accumulator, can be used to capture hydraulic energy stored in the head end of the hydraulic cylinder when retracting the hydraulic cylinder under an overrunning load. When operating in an energy recovery mode, pressurized hydraulic fluid from the head end of the hydraulic cylinder is discharged through the second pump and into the accumulator. If the pressure in the head end of the hydraulic cylinder is higher than that in the accumulator, the fluid drives the second pump like a motor, thereby creating a mechanical power output that returns energy to the engine. When extending the cylinder, pressurized fluid from the accumulator is supplied to the head end of the cylinder. A third of the three pumps is used as a pilot pump to provide pressurized fluid to control valves that regulate the flow of fluid between the cylinder, the second pump, and the accumulator.
Although the system of the '769 patent may recover some hydraulic energy when operating under an overrunning load, it may require large components and a greater number of components that may increase the size, complexity, and cost of the system. Because all of the fluid from the head end of the cylinder is discharged to the accumulator, the large size of the required accumulator may make packaging of the system difficult. Also, when the cylinder is retracted quickly under the force of gravity, a large quantity of fluid may be rapidly discharged from the cylinder, and the second pump/motor may need to be large to accommodate the rapid flow and large volume of fluid. The '769 patent system also requires an excessive number of hydraulic pumps, which may reduce the efficiency of the system and increase the control complexity and cost of the system.
The disclosed machine system is directed to overcoming one or more of the problems set forth above.